1. Field of the Invention
The present invention relates to an electrifying member for use in an electrophotograph apparatus. More specifically, this invention relates to an electrifying member that can have an electric voltage applied to it to electrify the surface of another member (to be electrified) so as to effect a desired process such as development and transfer. The present invention also relates to an electrophotograph apparatus employing said electrifying member.
2. Description of the Related Art
With a picture formation apparatus such as an electrophotograph apparatus (including a copying machine and an optical printer) or an electrostatic recording apparatus, conventionally, a corona discharge device has been used to act as an electrifying means for electrifying the surface of a picture carrier medium (a member to be electrified) such as a photosensitive medium or a dielectric medium.
A corona discharge device is useful as an electrifying means for uniformly electrifying the surface of a picture carrier medium so that said surface may have a uniform electric potential. However, since a high voltage electric power source is necessary for a conventional corona discharge device, and since ozone gas will occur due to corona discharge, a conventional corona discharge device is not as satisfactory as is expected in industry.
In order to cope with the above-mentioned problems associated with a conventional corona discharge device, there has been suggested a contact type electrifying device, which is capable of enabling an electrifying member (which has had a predetermined electric voltage applied thereto) to get close to or get in contact with the surface of a medium (to be electrified) so as to electrify its surface. With the use of such a contact type electrifying device, it is possible to reduce the voltage of the electric power source, and to inhibit the generation of ozone gas.
Usually, an electrifying member is required to possess a predetermined semiconductivity to prevent leakage which maybe caused by a damaged portion such as a pin hole formed on the surface of a medium (to be electrified) such as a photosensitive medium.
When a transfer roller is used in a picture formation apparatus such as a copying machine, such a transfer roller serves to transfer a toner picture from a picture carrier (such as a photosensitive medium, intermediate transfer medium or transfer drum) to a paper. In detail, a toner picture is developed on a photosensitive medium, the transfer roller is pressed against the photosensitive medium with a paper interposed therebetween. Then, by supplying electric charges having a polarity opposite to that of the toner of the toner picture, the toner of the photosensitive medium is absorbed by the paper so as to effect a desired transfer. Here, the quality of a picture is greatly affected by the density of the electric charges supplied to the paper. Namely, if there is a low electric charge density, the toner absorption force will be weak, presenting a problem called "black spots around line image" if using a dry paper. On the other hand, if there is a high electric charge density, it is likely that a problem called "blurring" will occur due to a possible electrification of reversed polarity. As a result, it is difficult to print a high quality picture.
Further, if the electric charge density is not uniform over a paper, it will be difficult to obtain a uniform concentration distribution of transfer using a black color, and some sand-like spots will occur on a transcribed paper. For this reason, it is desirable to provide a uniform electric charge density over the paper so as to obtain a uniform electric conductivity on the paper surface.
To meet the above requirements, it has been suggested to use an electrically conductive rubber roller which may be manufactured in the following manner.
Specifically, fine metal particles are coated with carbon black, graphite, or a metal oxide such as titanium dioxide, tin oxide, or a metal powder such as Cu or Ag, so as to obtain electrically conductive particles. Then, the electrically conductive particles are dispersed in a rubber material, thereby obtaining an electrically conductive rubber roller.
However, with the rubber roller with the above electrically conductive particles dispersed in it, a large amount of such electrically conductive particles have to be incorporated in the rubber roller in order to adjust the electric resistance. Therefore, it is difficult to obtain a uniform resistance distribution because it is difficult to form a uniform dispersion of the conductive particles throughout the whole volume of the rubber roller. As a result, it is difficult to obtain a uniform electrification effect, and it is likely to cause damage to the photosensitive medium due to a partial leak.
In addition to the use of the above electrically conductive particles to form an electrically conductive rubber, it is also possible to use ion type electrically conductive rubber. Such ion type electrically conductive rubber is formed by adding an ion type electrolyte such as LiClO.sub.4, NaSCN or LiCF.sub.3 SO.sub.4 so as to adjust its resistance.
This, however, will cause some other problems. Namely, since an electrolyte will drift to the surface of the rubber material with the passing of time, the electric resistance of the rubber roller will change and a photosensitive medium (provided as an outer layer) will be contaminated due to the electrolyte. Moreover, the resistance value will also change due to voltage application.
As to characteristics other than electric resistance, it is required that a transfer roller should have a low hardness in order to ensure an exact contact with a photosensitive medium, to alleviate possible damage to the photosensitive medium, and to ensure a stabilized movement of paper.
In order to ensure a low hardness for a transfer roller, it is usual to add a plasticizer or an oil to the rubber material so as to form a transfer roller with a sufficiently low hardness. However, since such a plasticizer or oil may also drift to the surface of a rubber roller, there is also a problem that a material (to be electrified) such as a photosensitive medium can be contaminated, and its resistance and hardness may change with the passing of time.
Japanese Patent Laid-open Nos. 58-87572 and 1-142569 have disclosed that using a hydrin rubber may provide good electrical conductivity for the same reason as discussed above. However, since an epichlorohydrin rubber has a large viscosity, it is impossible to obtain a sufficient non-adhesive effect even if it is blended with a non-polar polymer (as disclosed in Japanese Patent Laid-open No. 7-164571). Accordingly, after being used for a long period, toner will be absorbed on the surface of the electrifying member, hence damaging the electrical characteristics of the rubber roller.
On the other hand, epichlorohydrin rubber still remains in use in making a transfer roller, for it offers a good and uniform electrical conductivity without containing any electrically conductive additives. However, since an epichlorohydrin rubber will absorb a halogen gas such as hydrogen chloride occurring during a vulcanization process, it is difficult to obtain a uniform electric characteristic in a manufacturing lot. To cope with this problem, it has been suggested to use various acid neutralizing agents. One sort of acid neutralizing agent is a trilead tetroxide which has excellent reactivity, as disclosed in Japanese Patent Laid-open No. 6-161230. However, since it contains lead, the allowable amount of its use is usually restricted.